This invention relates to a printer employing a continuous form and, more particularly, to a detecting device for detecting traveling abnormalities such as skewing, meandering and the like of the continuous form.
Conventionally, there is known an image recording device utilizing a so-called electrophotographic system in which a surface of a photoconductive drum is exposed to light to form a latent image on the drum surface, toner is then applied to the latent image to develop the image, and the developed image is transferred onto a recording sheet material and is fixed by a fixing unit. Such image recording device is chiefly employed in a copying machine. In recent years, however, the image recording device is utilized in a printer and the like for printing out output from a computer.
In the copying machine, in general, cut sheets are used as the recording sheet material, and a heat-roll fixing system is utilized wherein the toner is fixed by heat as well as pressure. In addition, a pressure fixing system has recently been developed, which is low in electric power consumption and which does not require an amount of time for preheating the heat rolls.
In the printer, however, it is desired to use, as the recording material, a continuous recording form identical with that used in a conventional line-printer. The continuous recording form identical with the conventional one is a folded continuous recording form (hereinafter referred to simply as "continuous form") called a fan-folded form which has formed therein sprocket holes. Perforation is provided at each of the folded sections to enable sheet sections to easily be severed from each other.
When the above fixing systems are applied to the continuous form, the following problem might arise. That is, the continuous form clamped between the fixing rolls would skew or meander because of various factors such as poor initial biting of the continuous form into the nip between the fixing rolls, unevenness or nonuniformity in thickness of the continuous form, elongation of the continuous form due to absorption of moisture, and the like. If such skewing or meandering occurs, the biting position of the continuous form with respect to the fixing rolls consecutively varies laterally, so that the side edge of the continuous form finally reaches the lateral end of the nip between the fixing rolls. This causes creases in the continuous form, resulting in defective fixing of the image and in traveling troubles of the continuous form. In view of such problems, an arrangement has been proposed in which a tension-applying mechanism is provided for applying a tension to a portion of the continuous form extending between the photoconductive drum and the fixing roll pair, to uniformize the state of the continuous form to be bitten into the nip between the pair of fixing rolls. By this tension-applying mechanism, it is made possible to prevent meandering or skewing of the continuous form, and to automatically restore the continuous form to the regular position even if such meandering or skewing occurs.
However, traveling troubles incapable of being restored sometimes occur due to factors such as forces applied to the continuous form from the outside, excessive elongation and shrinkage of the continuous form, a malfunction of the transport system, severance of the continuous form, and the like.
If the continuous form continues to travel while occurrence of the above troubles is maintained, the continuous form is bitten into the transport system and the drive system. This results in jamming of the continuous form so that a load is applied to the device. Thus, there arises a serious problem that, in the worst case, the above troubles result in malfunction or destruction of the device. Therefore, it is necessary to immediately halt operation of the device. However, manual operation is slow in response and cannot adequately deal with the troubles.